libhdfs++: Add a synchronization interface for the GSSAPI

Bits of the GSSAPI that Cyrus Sasl uses aren't thread safe. There needs to be a way for a client application to share a lock with this library in order to prevent race conditions. It can be done using event callbacks through the C API but we can provide something more robust (RAII) in the C++ API.

Proposed client supplied lock, pretty much the C++17 lockable concept. Use a default if one isn't provided. This would be scoped at the process level since it's unlikely that multiple instances of libgssapi unless someone puts some effort in with dlopen/dlsym.

class LockProvider
{
  virtual ~LockProvider() {}
  // allow client application to deny access to the lock
  virtual bool try_lock() = 0;
  virtual void unlock() = 0;
}